The shaft arrangement most commonly used for fastening grindstones in grinders used to produce fibrous pulp consists of a heavy shaft extending through the grindstone and through the grinder bearings on each of its sides. To retain the grindstone, the shaft is provided on both sides of the stone. There is a strong screw thread with a righthand thread on one side and a lefthand thread on the other side. Steel discs rigidly attached to nuts fitted on these threads are pressed from both sides of the stone hard up against it. Due to the right and lefthand threads, the pressure of the discs on the stone will increase with an increase in the load to which the stone is subjected. A device of this kind is shown in Swedish Pat. No. 80,084. In Swedish Pat. No. 331,589 an arrangement is shown where a grindstone with a shaft of the type in common use extending through the grindstone has a metallic cylinder coaxially attached to the shaft by means of flanges at each of its ends. The ceramic sectors of the grindstone that perform the grinding work are attached to the shell of the metallic cylinder.
The known arrangements, referred to above, have certain disadvantages. An important disadvantage is that the shaft is weakened by the threads. Fracture of the shaft occurs in the fractural impressions on each side of the stone. A contributory cause of fracture may be that both side faces of the grindstone itself are not truly parallel. Spacers between the steel discs and the side faces of the stone are not made in such a way that the deviation of parallelism is fully compensated, so parts of the threads will be subjected to a high shear stress at the same time the shaft is subjected to a bending stress. The combined shear and bending stresses produced by the feed pressure will create such high stress peaks that after a certain time in operation it will fracture from fatigue. The high torque that the grindstone transmits to the shaft through the threads subjects the shaft to a high tensile stress which contributes to shaft fracture.
A second disadvantage of prior arrangements is exemplified when a used grindstone is removed and a new one is installed. It is not usually possible to center the new stone when it is mounted on the shaft. For that reason, the stone must be machined on the grinder and then sharpened before the grinder can be returned to service.